With the pursuit of recent green growth, high attention has been paid to the environment. As such, the reduction in emission of carbon dioxide for preventing global warming is becoming an international interest. This carbon dioxide emission reduction draws attention because of a possibility of creating a commercial value called carbon emission rights in addition to the prevention of global warming.
In the Europe Climate Exchange located in Amsterdam, Netherlands on the largest scale in the world, the carbon emission rights were traded at an amount of four hundred million tons or more in 2006. The trade of the carbon emission rights is increasing 50% or more each year, and thus the attention to the carbon emission rights is increasing.
In the field of pavement technology of asphalt concrete (asphalt mixture, abbreviated to “ascon”), technology for reducing the emission of carbon dioxide in harmony with the atmosphere has been actively reviewed.
Among them, the most attracting technique in practicality and scale is a pavement technique using a low carbon asphalt mixture capable of reducing mixture and compaction temperatures upon asphalt concrete pavement construction.
Typically, the asphalt mixture is produced by charging asphalt, aggregate, mineral filers, etc. into an asphalt mixing plant, and then heating and mixing these materials.
In this manner, the asphalt mixture is produced by a heating process at a high temperature of 160° C. to 180° C., and undergoes cooling to room temperature in a spreading and compacting process for the pavement.
Here, the reason for which the high-temperature heating process is required to produce the asphalt mixture is that it is necessary to liquefy the asphalt such that the asphalt acts as a binder of the aggregate.
Generally, the asphalt mainly used for asphalt pavement is petroleum based asphalt (in which straight asphalt is mainly used and typically called an “asphalt binder”), which exists in a black solid state at room temperature.
As such, the asphalt is heated and liquefied. An adhesive strength of this liquefied asphalt is used for binding of the aggregate.
Thus, the asphalt mixture used for the asphalt pavement is produced as a hot-mix asphalt (HMA) mixture.
To produce the asphalt mixture, a great deal of energy is required to heat the asphalt mixture to a high temperature. Even in construction, the asphalt mixture emits a large quantity of harmful gases such as carbon dioxide (CO2), etc.
Further, the high-temperature asphalt mixture spread and compacted in the event of the asphalt pavement delays a time to open traffic in proportion to a time required to be cooled close to room temperature, and causes workers to be exposed to a risk of a safety accident.
To solve these problems, studies have recently been actively made of a warm-mix asphalt (WMA) mixture, which can be mixed and compacted at a lower temperature than the existing HMA mixture.
In other words, studies have been actively made for reducing the mixture and compaction temperatures of the asphalt and the aggregate to thereby reduce the emission of carbon dioxide caused by the pavement of the asphalt mixture.
Pavement technology using this WMA mixture has the following merits compared to that using the HMA mixture, because the WMA mixture can be produced at a temperature of about 110° C. to 140° C., which is 30° C. to 40° C. lower than a temperature required to produce the HMA mixture.
1) It is possible to inhibit a variety of harmful gases from being generated in the process of producing and spreading the asphalt mixture.
2) It is possible to reduce petroleum fuel as a principal component of green house gas by about 30% in the process of producing the asphalt mixture.
3) It is possible to rapidly reopen traffic by reducing a curing time after the asphalt mixture is applied.
4) It is possible to secure safety of workers because no harmful steam or odor is generated in a road construction field.
A core mechanism of the pavement technology of the WMA mixture is to improve fluidity of the asphalt, i.e. to reduce the viscosity of the asphalt as the binder of the aggregate such that an optimal viscosity of the asphalt is obtained at a lower temperature than the HMA mixture, and that an optimal compaction rate of the asphalt is obtained at a lower temperature.
This technology for reducing the viscosity was proposed in 1956 first by Ladis H. Csanyl of Iowa State University, USA, in which the viscosity of the asphalt is reduced by forcibly injecting steam into the asphalt so as to form foam asphalt caused by moisture and air in the asphalt and thus to reduce an internal stress of the asphalt.
This technology has recently been developed in Europe since 2000 to technology for forcibly emulsifying the asphalt to reduce the viscosity of the asphalt by injecting water instead of steam, and technology for producing a foam-WMA mixture that adds zeolite instead of water to discharge contained moisture into water when the zeolite reaches a predetermined temperature (e.g. about 100° C. or more), and thus emulsifies the asphalt.
Meanwhile, the National Center for Asphalt Technology (NCAT) has reported the WMA mixture in 2005 for which polyethylene wax, Sasobit, produced by a Fischer-Tropsch synthesis method, is used.
Sasobit is a compound having a molecular weight of several hundreds, which is in a solid state at room temperature and is changed into fluid at a higher temperature. This wax is added to the asphalt, thereby abruptly reducing the viscosity of the asphalt above its melting point and experiencing solidification below its melting point. As such, Sasobit is used as an additive for enhancing an effect of the WMA mixture.
Typically, the polyethylene wax may be produced in various methods. Among these methods, one may involve purifying by-products obtained in the process of producing and processing polyethylene to produce the polyethylene wax, and another method may involve pyrolizing polyethylene to produce the polyethylene wax. Further, there is a method of polymerizing a single molecular material to produce wax molecules, which is a representative example of using the Fischer-Tropsch synthesis method as in Sasobit.
The following three methods are used to produce the polyethylene wax.
(1) In the first method, the polyethylene wax is produced from a low-molecular weight by-product obtained in the process of producing a polyethylene resin.
(2) In the second method, the polyethylene wax is produced by pyrolizing a polyethylene resin to reduce its molecular weight.
(3) In the third method, the polyethylene wax is produced by increasing the molecular weight of a single molecular material, for instance, in the Fischer-Tropsch synthesis method.
In general, the physical properties of the polyethylene wax are known to be decided by the line shape of a main chain and a molecular weight in a chemical structure thereof.
In detail, the greater the linearity of the main chain of the polyethylene wax, the higher the molecular regularity. Thereby, crystallinity is increased, and thus the physical properties such as melting point, hardness, etc. are increased. It is known that it is possible to produce the polyethylene wax having excellent physical properties such as melt viscosity when the molecular weight of the polyethylene wax is great.
The polyethylene wax obtained from the by-products obtained in the process of producing the polyethylene resin or by pyrolizing the polyethylene resin, is low in the linearity of the main chain thereof compared to that produced by the Fischer-Tropsch synthesis method, and thus its molecular weight must be increased to improve its physical properties. In this case, the melt viscosity of the polyethylene wax is increased, so that the polyethylene wax cannot be used as the additive of the WMA mixture, and low-temperature stiffness of the polyethylene wax is increased to reduce low-temperature crack resistance of the binder.
In contrast, since the Sasobit wax produced by the Fischer-Tropsch synthesis method has its molecular structure in which a normal alkane (n-alkane) structure takes 90% or more, the Sasobit wax has a long chain aliphatic hydrocarbon structure having fewer branches than the wax produced by any other method.
Thus, the Sasobit wax has low viscosity and high-temperature physical properties, so that it shows physical properties more suitable as a low-carbon additive than any other polyethylene wax.
Due to its low viscosity, the Sasobit wax produced by the Fischer-Tropsch synthesis method has been used as a low-carbon additive, which is very effective for the WMA.
However, although the polyethylene waxes including this Sasobit wax are very excellent in the high-temperature physical properties such as permanent deformation resistance (called rutting resistance), after long-term aging of the asphalt, the asphalt shows an increase in stiffness and a decrease in the rate of change of stiffness (called m-value), and thereby its flexibility is reduced. Thus, it can be seen that the low-temperature physical properties of the asphalt are overall reduced.
As such, the polyethylene waxes including the Sasobit wax have a possibility of reacting as a factor that causes a low-temperature crack of the asphalt mixture (mainly occurring in winter), which is a crack that is originated from the upper portion of a paved layer to be propagated to the lower portion by the imbalance of temperature distribution of the paved layer of the asphalt mixture, and is characterized by transverse generation of the pavement.
Thus, it is necessary to solve a problem on the decrease of the low-temperature physical properties in the WMA mixture using the Sasobit wax as the additive.